As a winding or a wiring material for high-frequency equipment, the so-called litz wire (6), as shown in FIG. 2, which consists of a prescribed number of twisted separately-insulated strands formed by providing an enamel film (8) on each of conductors (7) is used. The litz wire is mainly used in low-voltage parts out of high-frequency equipment. However, when the litz wire is wound into a coiled shape, the cross-section of the conductor is likely deformed, making the aligned winding difficult. Accordingly, in such a coil, the deformation of the cross-section of the coil affects adversely the properties of the high-frequency equipment. On the other hand, when the litz wire is used in a part where a high-frequency surge is applied, a high voltage is applied to the ends of the coil or between the layers. In such a type, means of securing reliability, such as additional insertion of insulating paper between the layers, is required because, in the conventional litz wire, the insulating properties are determined by the insulating properties of the insulated strands themselves.
To solve such problems, the inventors of the present invention proposed an improved insulated wire for high-frequency equipment, as shown in FIG. 1. Referring to FIG. 1, the insulated wire 1 is composed of plural enamel-insulated strands 2 (formed by providing an enamel film 5 on each outer peripheral surface of conductors 4) put together, and covering layer 3 on the other periphery of the strands 2 formed by extrusion-coating of a thermoplastic polyester-series resin that is modified with an ethylene-series copolymer having carboxyl groups (JP-A-4-10305 ("JP-A" means unexamined published Japanese patent application)). This insulated wire overcomes the defects of the conventional litz wire, and it has an advantage that it makes soldering possible only by dipping a terminal of the wire in a solder bath. This insulated wire is already used practically.
However, in recent years, as high-frequency equipment is reduced in size and high performance is required, in some applications, heat resistance of the wire becomes unsatisfactory, and such applications are sharply increased. Therefore, improvement in heat resistance is becoming quite an important subject.
Thus, to obtain such an insulated wire, the improvement is attempted mostly by using alternative high heat-resistant materials.
However, the insulated wire provided with an insulating layer of a high heat-resistant material is accompanied by the problem that the removal (peeling) of the insulating layer is very difficult when the insulating layer is removed for connection of terminals. This lowers the solderability at the terminal part of the insulated wire and acts as a major factor against high reliability of the insulated wire to be developed. In other words, when the insulating layer is removed with a chemical or mechanically, if a high heat-resistant material is used for the material of the insulating layer, the removal conditions become severe, causing the chemical to remain or increasing flaws due to the mechanical removal, thereby adversely affecting the connection of terminals. On the other hand, if it is tried, eluding this problem, only to increase the reliability of the connection of terminals, improvement of the heat resistance of the insulated wire is conspicuously impeded.
Further, as the diameter of the insulated wire is reduced, the conventional operation method of removing the insulating layer is made increasingly difficult and causes the reliability of the insulated wire to be lost.